It has long been known that the effective resistivity of certain metals was sometimes substantially eliminated when the metal was exposed to low temperature conditions. Of particular interest were the metals and metal oxides which can conduct electricity under certain low temperature conditions with virtually no resistance. These have become known as superconductors. Certain metals, for example, are known to be superconductive when cooled to about 4.degree. on the Kelvin scale (.degree.K.), and certain niobium alloys are known to be superconductive at about 15.degree. K., some as high as about 23.degree. K.
Discovery of superconductivity in the system La-Ba-Cu-O (J. G. Bednorz and K. A. Muller, Zeit. Phys. B 64, 189-193 [1986]) has stimulated the search for other systems, particularly with a view to substituting other elements for the rare earths (RE) used in the earlier materials. For example, replacement of RE by Bi and Tl has been reported. (See M. A. Subramanian et al.,) Science, 239, p. 1015 (1988); L. Gao et al., Nature, 332, pp. 623-624 (1988).
Wang et al., Comparison of Carbonate, Citrate, and Oxalate Chemical Routes to the High-T.sub.c Metal Oxide Superconductors La.sub.2-x Sr.sub.x CuO.sub.4, Inorg. Chem. 26, 1474-1476 (1987) discloses a carbonate precipitation technique. The precipitant was K.sub.2 CO.sub.3. According to the paper, it was necessary to wash the precipitate repeatedly, an obvious disadvantage in production work. Washing was necessary because potassium adversely affects superconductivity properties of the finished materials. Repeated washing removes strontium, a highly detrimental loss in the process.
From the technical viewpoint it may seem obvious that co-precipitated carbonates would provide enhanced homogeneity. However, the technical solution to the problem encounters serious difficulties. Thus the Wang et al process, using potassium carbonate (or sodium carbonate) necessitated numerous washings and apparently left detectable amounts of alkali in the ceramic base even so. As noted serial washings remove Sr, and would be unworkable in my process. Nor is it merely sufficient that the carbonate be derived from a cation that would burn off completely. For example, ammonium carbonate does not work, because a pH below 7 is required to prevent formation of copper tetraamine, but under these conditions bicarbonate ion is formed, with consequent formation of strontium bicarbonate, which, being slightly soluble, disrupts the desired stoichiometry. Quaternary ammonium carbonates, on the other hand, form the desired metal carbonates simply and cleanly as a coating on Sb.sub.2 O.sub.5 particles without troublesome side-formation of complexes or coordination compounds, with firm and precise retention of the intended stoichiometry. The coated particles are readily recovered for further processing.
In the prior art, Bi-Pb-Sb-Sr-Ca-Cu-O superconductors have been made by heating together Bi.sub.2 O.sub.3, CuO, SrCO.sub.3, CaCO.sub.3, PbO and Sb.sub.2 O.sub.5 in air, at, e.g. 830.degree. C. for 15 hours, followed by grinding, pelletizing, sintering at 880.degree. C. for 12 hours, followed by furnace cooling in air. Processes of this type are disclosed in the following preprints:
Mao, X., et al, The Influence of Pb composition on the Upper Critical Magnetic Field of Bi.sub.1.9-x Pb.sub.x Sb.sub.0.1 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y system.
Liu, H. et al, Zero Resistance at 132.degree. K. in the Multiphase System of Bi.sub.1.9-x Pb.sub.x Sb.sub.0.1 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y with x=0.3, 0.4.
Liu, H., et al, Superconducting Transition above 160.degree. K. in Bi-Pb-Sb-Sr-Ca-Cu-O System.
Liu, H., et al, Superconducting Properties in (Bi.sub.2-x-y Pb.sub.x Sb.sub.y)Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.z System (x=0, 0.1, 0.3, 0.5; y=0, 0.1).
A published article is similar but omits PbO:
Liu et al, The Superconducting Properties in Bi.sub.2-x Sb.sub.x Sr.sub.2 - Ca.sub.2 Cu.sub.3 O.sub.4 Compounds (x=0.05, 0.1 0.15, 0.2) Physica C 156, pp. 804-806 (1988).